1. Field of the Invention
The present invention relates to new synthetic zeolites incorporating alkaline earth cations and routes for preparing those zeolites by hydrothermal alteration of zeolite Pi. New methods for producing P1 zeolites are also identified.
2. Description of the Related Art
Heulandite-type zeolites are the most abundant natural zeolites and are frequently found in volcanic rocks, altered volcanic tuff deposits and deep-sea sediments.
The topology of heulandite-type zeolites is characterized by a two-dimensional void structure consisting of pores composed of 8 tetrahedral atoms in the 100! direction that intersect pores composed of 10 tetrahedral atoms in the 001! direction. Heulandite can be distinguished from clinoptilolite on the basis of chemical composition. If the Si/Al ratio .ltoreq.4 and the cation content is such that Ca&gt;(Na+K), then the zeolite is referred to as heulandite; otherwise, it is called clinoptilolite. Although heulandite and clinoptilolite have the same maximum framework topology, the natural conditions under which they are formed, as well as their physical and thermal properties are markedly different.
Natural heulandites and clinoptilolites have found applications in ion-exchange and separation processes. However, their use as catalysts is limited by the presence of relatively large amounts (on the order of 0.5 weight %) of impurities such as iron and titanium. Such impurities are commonly found in natural samples as oxides and hydroxides both in the zeolite framework as other crystalline phases or amorphous tuffs associated with a particular occurrence of the zeolite.
One potential catalytic application of an impurity-free synthetic zeolite with the heulandite topology is that of the isomerization of 1-butene to isobutene as suggested by the recent work of Woo, et al. The development of an efficient catalyst for the production of isobutene could have great industrial implications due to the versatility of isobutene in the manufacture of a number of chemicals particularly that of methyl tert-butyl ether (MTBE) as an octane booster for gasoline.
Heulandite has proven exceedingly difficult to synthesize in the laboratory. To date, there are two published synthesis routes to heulandite. In 1960, Koizumi and Roy reported the synthesis of a heulandite-type zeolite from the composition CaO.Al.sub.2 O.sub.3.7SiO.sub.2.5H.sub.2 O at temperatures between 250.degree. C. and 360.degree. C. and a pressure range of 15000 to 37000 psi. In 1981, Wirsching obtained heulandite by hydrothermal alteration of rhyolitic glass under the action of CaCl.sub.2 solutions at temperatures of 200.degree. C. to 250.degree. C. and reaction times of around 80 days. Additionally, some syntheses for clinoptilolite zeolites have been reported.
Similarly, other zeolites are either difficult to produce, or have not been produced synthetically at all. For example, brewsterite is a naturally occurring, though rare, zeolite, for which a synthetic preparation has not previously been reported. It has a pore system described by intersecting channels consisting of pores composed of eight tetrahedral atoms in the 100! and 001! directions.
Harmotome is another rare zeolite of hydrothermal origin which has the phillipsite (PHI) topology and is characterized by three dimensional channels consisting of pores composed of eight tetrahedral atoms. The dominant cation in the zeolite is Ba.
Epistilbite is another rare zeolite of hydrothermal origin, which has previously been produced by hydrothermal treatment of rhyolytic glass at 250.degree. C. and from powdered SiO.sub.2 glass at 250.degree. C. It has a structure characterized by intersecting channels composed of eight and ten tetrahedral atoms.
Since these naturally occurring materials are rare, but so potentially useful, it would be advantageous to identify less rigorous and therefore less expensive routes for the production of greater quantities of these materials. Particularly of interest would be routes which would produce materials with few or no impurities.